This invention relates to portable radio, telephones and in particular to such telephones for communication with base stations in a digital cellular radio telephone system employing transmission by a plurality of carrier frequencies in frames, such as TDMA frames, each consisting of a predetermined number of time slots. The invention also relates to a method of operation of such radio telephones, frequently called handsets. The invention is concerned with the allocation of a channel (that is a combination of a carrier frequency and time slot) to a portable radio telephone when a connection is first required (xe2x80x9ccall set-upxe2x80x9d) or when a change in channel is required (xe2x80x9chandoverxe2x80x9d) during a call to maintain call quality. Handover can either be to a different channel at the same base station (intracell) or to a different channel at a different base station (intercell).
Most of the currently manufactured DECT base station equipment only contains a single transceiver, and is therefore unable to open more than a single communication channel on different frequency carriers at the same instant. This restricts single transceiver base stations to only using a single time slot at any one time, effectively excluding the other carriers on this time slot and thereby turning them xe2x80x9cblindxe2x80x9d. The DECT standards have foreseen this and include the blind slot information message to inform the handset of time slots it should avoid using. However, this information is not totally reliable, particularly on intercell handover when it is very difficult to obtain the blind slot information of any base station other than the one that the handset is connected to unless a separate xe2x80x98observationxe2x80x99 channel is opened. In addition to slots xe2x80x9cblindedxe2x80x9d by the base station, a handset will be unable to; switch communication channels to a slot immediately adjacent to that which it is using.
This invention aims to provide a portable radio telephone, and a method of operation, employing a dynamic channel assignment algorithm that gives very robust performance, regardless of the availability and accuracy of the blind slot information. In addition to this, if an identical algorithm is deployed on every portable radio telephone in the system, further improvements in call blocking and call quality are likely.
According to one aspect of the invention there is provided a portable radio telephone for communication with base stations in a digital cellular radio telephone system employing transmission by a plurality of carrier frequencies in frames each consisting of a predetermined number of time slots, wherein the portable radio telephone comprises processing means operative to select for the radio telephone a communication channel, that is a combination of carrier frequency and time slot, by excluding any channel in a time slot where a channel selection attempt at a base station has failed.
It will be appreciated that the exclusion of a time slot will be temporary and that a particular slot will be xe2x80x9cunblindedxe2x80x9d by subsequent events. Preferably, the exclusion of a slot prevails for a number of unsuccessful channel selection attempts.
Preferably, on call handover from a current channel to a new channel the processing means additionally exclude any channel in the three adjacent time slots centred on the current channel.
The portable radio telephone may comprise storage means for storing a blind slot mask representative of the slots in each frame which are excluded, the remaining slots being available for selection of a channel.
Once channel selection has been unsuccessfully attempted in each of the remaining slots, the blind slot mask can be reversed so that those slots that have not previously been tried are rendered available for possible selection of a channel.
The processing means may be operative to check the slots for availability in a predetermined sequence which may correspond to the temporal sequence of the slots or to the reverse of the temporal sequence of the slots, until an available slot is found and a channel therein is selected.
In the preferred embodiment to be described, slots are checked for availability in sequence, starting from the first slot in the frame allocated for handset transmission and progressing to the final slot, before returning to the first slot.
According to another aspect the invention provides a method of communicating between a portable radio telephone and base stations in a digital cellular radio telephone system employing transmission by carrier frequencies in frames each consisting of a predetermined number of time slots, comprising selecting for the radio telephone a communication channel, that is a combination of carrier frequency and time slot, by excluding any channel in a time slot where a channel selection attempt at a base station has failed.
According to a yet further aspect there is provided a portable radio telephone for communication with base stations in a digital cellular radio telephone system employing transmission by a plurality of carrier frequencies in frames each consisting of a predetermined number of time slots, wherein the portable radio telephone comprises processing means operative to select for the radio telephone a communication channel, that is a combination of carrier frequency and time slot, such that successive attempts at channel selection are made in respective ones of the predetermined number of time slots.